Chemically tunable acoustic phonons in 2D layered MoO3

Intercalation of zero-valent metals is a unique chemical strategy to alter the chemical and physical properties of two-dimemsional (2D) layered materials. Using intercalation of an entire series of elements (Ag, Au, Bi, Co, Cr, Cu, Fe, Ge, In, Mn, Mo, Ni, Os, Pd, Pt, Rh, Ru, Sb, Sn, and W), we demonstrate full chemical tunability of the electronic bandgap, structure, and phonons of α-MoO₃. Optical chemochromism is demonstrated with intercalants changing the color of MoO3 from transparent to dark blue (Ge-, In-, Mo-, Os-, Ru -MoO₃) to black (Ni-MoO3) and orange (Ag-MoO₃). An increase in absorption > 600 nm is found for some intercalants (Cr-, Ge-, In-, Mn- Os-, Ru- MoO₃) associated with intercalation-driven chemochromism in these materials.

Using Brillouin laser light scattering, the quantized acoustic phonons in 2D layered pristine MoO3 and intercalated MoO₃ are measured at GHz frequencies. As MoO3 crystals are < 200nm in thickness and the phonon wavelength is comparable to material thickness, scattering of acoustic phonons yields multiple phonon branches. The entire angular acoustic phonon dispersions of MoO₃ are measured. Combining these measurements with Lamb wave theory, the complete elastic tensor of nine independent elements (c_ij) is determined in pristine and intercalated MoO₃. Measurements also yield the refractive index, bulk modulus, Young’s moduli (E₁₁, E₂₂, E₃₃), Poisson’s ratio, and the directional longitudinal and transverse sound velocities. Combining all these measurements provides a unique picture into how intercalation can be used to control material properties. Results suggest a surprising degree of complexity with competition amongst multiple effects, electronic and structurally, of the intercalant.